The hip joint is a ball-and-socket joint which movably connects the leg to the torso. The hip joint generally comprises a femoral component (i.e., the neck and ball at the top end of the femur) and an acetabular component (i.e., the acetabular cup formed in the pelvis).
In many situations, a femoral component of a hip joint may need to be replaced by a prosthetic device. By way of example but not limitation, a femoral component of a hip joint may need to be replaced by a prosthetic device because of injury (e.g., a fracture of the femoral neck), degenerative disease (e.g., osteoarthritis, rheumatoid arthritis, post-traumatic arthritis, avascular necrosis of the femoral head, etc.), developmental pathologies (e.g., Perthes disease, developmental or congenital hip dysplasia, etc.), etc.
Such replacement of a femoral component of a hip joint with a prosthetic device typically includes surgical removal (or “resection”) of the compromised femoral neck and head, followed by reconstruction using a femoral prosthesis, as will hereinafter be dismissed. In many cases, the acetabular cup may also be replaced by a prosthetic device. Where both the femoral component and the acetabular component are replaced by prosthetic devices, the procedure is commonly referred to as a “total hip replacement” (THR), or a “total hip arthroplasty” (THA), or a “bipolar hip reconstruction”; and where only the femoral component is replaced by a prosthetic device, the procedure is commonly referred to as a “hemiarthroplasty”, or a “unipolar hip reconstruction”. In any case, the surgical goals of replacing the injured and/or diseased bone and cartilage surfaces with a prosthetic device is to reduce pain, improve range of motion, improve weight-bearing ability, increase mobility and, in turn, decrease the risk of recumbency-related complications.
The femoral prosthesis generally comprises a femoral stem, a femoral head (or “ball”), and a femoral neck. The femoral stem is received within the intramedullary space of the proximal femur. The femoral head is designed to articulate within the acetabular component of the hip joint (i.e., in either a prosthetic acetabular cup or the native acetabular cup). The femoral head is connected to the femoral stem by the femoral neck, and the femoral head is typically locked to the femoral neck via a Morse taper mechanism. The femoral neck is typically formed integral with the femoral stem, although in some cases they may comprise separate components which are united during surgery.
The femoral stem is generally constructed from a high-strength metal alloy or stainless steel, and its outer surface is often treated and/or configured so as to promote bony ingrowth, bony ongrowth, or bony interdigitation.
The femoral stem can be fixed within the intramedullary space of the proximal femur by either using bone cement (e.g., methylmethacrylate) or in a cementless press-fit manner (hereinafter referred to simply as “press-fit”).
The present invention is directed to press-fit femoral stems.
Press-fit implantation is commonly referred to as a “biologic reconstruction” in the sense that the host bone will eventually grow into, and/or onto, the femoral stem's outer surface over a period of weeks to months. In the immediate post-operative period, and until such time that biologic reconstruction (i.e., bone growth) can provide bony implant security, the stability of the femoral stem is dependent upon radially directed hoop-stresses which are created when the implant is forcefully wedged into the intramedullary space. In this respect it should be appreciated that the stability of the femoral stem is vital in order to ensure long-term prosthetic functionality and for maintaining equal leg length (relative to the contralateral limb). Early loosening of the femoral stem, and/or selecting an under-sized femoral stem, may lead to subsidence (or “sinking”) of the implant further within the intramedullary space of the proximal femur, thereby resulting in leg length inequality, altered hip biomechanics, and gait abnormality. Early loosening of the femoral stem is also associated with pain and, frequently, with the need for subsequent revision hip surgery.
Conversely, an over-aggressive impaction of the femoral stem (i.e., selecting an over-sized femoral stem) may result in exceeding the hoop-stress capacity of the proximal femoral bone, thereby resulting in a fracture of the femoral bone. Such an occurrence will, at a minimum, require additional surgical attention and may also require additional weight-bearing restrictions for the patient. If unrecognized, this fracturing of the bone may also result in post-operative subsidence of the femoral stem (and hence sub-optimal function of the joint and substantial pain for the patient) and may necessitate revision surgery.
In addition to the foregoing, stress shielding is a phenomenon where normal, physiological stress forces travel unequally across an implant and, in so doing, may bypass a region of bone such as the proximal femur. Since substantial bone density and substantial bone strength are enhanced by the presence of stress, the occurrence of stress shielding can result in decreased bone density and decreased bone strength in the area of the bone which is stress shielded. This phenomenon, commonly referred to as Wolff's law, is well known by those skilled in the art. Because of this phenomenon, it is generally desirable to minimize stress shielding when deploying a femoral stem in a bone, so as to maintain bone strength/integrity in the region adjacent to the implant and thereby avoid fractures through the region.
Current press-fit femoral stem designs typically comprise either (i) a “proximally coated” (or a “proximally porous-coated”) stem, or (ii) a “fully coated” (or a “fully porous-coated”) stem.
The “proximally coated” stem design permits biologic fixation via bone ingrowth, or ongrowth, along the more proximal region of the stem. The proximally coated stem design benefits from minimizing bone loss secondary to stress shielding, however, the initial stem stability is completely dependent upon the hoop-stresses created when the implant is forcefully wedged into the intramedullary space of the proximal femur.
Conversely, the “fully coated” stem design promotes bony ingrowth, or ongrowth, along the entire length of the femoral stem. The fully coated stem design is intended to provide diaphyseal (i.e., more distal) bone fixation and does not depend upon the hoop-stresses resulting from wedging the bone into the more proximal region of the femur. This fully coated stem design establishes an initial wedging, or “scratch fit”, along the length of the femoral diaphysis. The fully coated stem design frequently includes a collar, which prevents implant subsidence and serves to mark the desired longitudinal implant height within the intramedullary space of the proximal femur. The fully coated stem design benefits from excellent initial and long term fixation but, conversely, can be associated with stress shielding of the proximal femur. In addition, removal of this type of implant, as may be required in certain situations such as infection, can be more technically challenging than with a proximally coated stem design and can, in some instances, result in a greater degree of iatrogenic bone loss.
Thus, there is a need in the art for a new femoral prosthesis which maximizes the benefits of press-fit technology while minimizing the disadvantages and inadequacies of the prior art previously described.